The study of reliability of progressive slope failure is presented considering the variability and correlativity of shear strength parameters of soil. Soil mass above slip surface was divided into n slices according to equal arc length. The safety margin of every slice is assumed to be normal distribution and the safety margin of two next slices is bivariate normal distribution. The probability of progressive slope failure and expected value of leak length on the top of slope are calculated by the probability of propagation of slice failure. The results of an example are discussed.